The invention relates to a ballast for igniting and feeding a high-pressure gas discharge lamp, which ballast comprises means for supplying the lamp during an ignition period (ti) with an AC voltage at an ignition frequency (fi), means for supplying the lamp with an AC voltage at a glow frequency (fg) during a glow period (tg) following the ignition period (ti), means for at least repeating the start-up sequence of ignition and glow periods (ti, tg), and means for feeding the lamp during an operating period (tb) following the last start-up sequence. Mention is made here of an ignition period (ti) and a subsequent glow period (tg), however, this should be construed such that, in practice, a clear distinction between these two periods cannot always be made, whereby the ignition frequency (fi) and the glow frequency (fg) can be variable and gradually blend with each other, or they may even be equal to each other.
The invention also relates to a projection unit provided with such a ballast.
Such a ballast is described in the international patent publication WO-99/35890 and can be used to start-up or ignite and feed a high-pressure mercury-vapor gas discharge lamp having, for example, a rated power of approximately 100 W, which is used predominantly as a (slide, sheet or LCD-screen) projection lamp. Such a lamp comprises a bulb or tube of quartz glass as the discharge vessel, which is provided on two sides with an electrode, which is made, for example, of tungsten and which has a solid core around the end portions of which a wire is wound. The discharge vessel contains a quantity of mercury. During the ignition period (ti), a high voltage is generated by means of a high-frequency voltage source, resulting in a discharge arc between the electrodes. During the subsequent glow period (tg), the frequency is reduced, so that the impedance in the circuit decreases and the current flowing through the lamp increases. As a result, the electrodes heat up more rapidly. The increase in temperature causes the mercury to evaporate, so that the necessary mercury vapor pressure is built up. If the electrodes are at a sufficiently high temperature, the ballast goes over to the stable operating period (tb), which, in the case of the known ballast, has a comparatively low operating frequency (fb). If, however, it is detected through measurements that the lamp is not burning after this start-up sequence, the start-up sequence (ti, tg) is automatically repeated until the lamp does burn. If the lamp still does not burn after the start-up sequence has been repeated a number of times, the start-up cycle is interrupted.
It is desirable for the discharge arc to develop along the shortest path between the two electrodes so as to make the electrodes heat up entirely. A problem concerning the start-up of such a lamp is that the discharge arc initially often develops between the rear sides of both electrodes, where no wire is wound, so that the cross-section of the electrode is smaller at said location. The mercury tends to deposit at this part, which leads to a better conduction and hence enhances the formation of a discharge arc from this location. Furthermore, the electrodes may be covered with salts, which can counteract the development of the desired discharge arc. As a result, an elongated, curved discharge arc may be formed between the rear sides of the electrodes, which may subsist also during the stable operating period, which does not lead to optimum results since the electrode is heated only partly instead of bodily because only the part of the electrode through which current flows is heated. The lamp may spontaneously go out during the start-up time because the discharge arc is too long and the temperature of the electrodes is too low, in which case the lamp has to be put into operation by hand. This problem occurs, in particular, with lamps having a high rated power and heavier electrodes, which is the reason why they have not been widely used hitherto. Lamps having such a high power comprise a plurality of layers of wound wire, leading to comparatively wide electrode end portions as compared to lamps having a lower power, so that the above problems are more likely to occur. There is, however, a need for projection lamps having such a high power, which can be used for projection representations at ambient light conditions.
It is an object of the invention to provide a measure for a ballast which can be used to ignite and feed a high-pressure gas discharge lamp, which measure counteracts the above-mentioned drawbacks and problems.
To achieve this, the ballast of the type mentioned in the opening paragraph additionally comprises means for interrupting the power supply to the lamp in a start-up sequence, which is not the last start-up sequence, during an interruption period (td) following the glow period (tg). The invention thus includes an intentional interruption of the power supply during a short period of time. Preferably, the interruption period (td) is long enough to substantially de-ionize the gas contained in the lamp.
Preferably, the interruption period (td) ranges between 50 ms and 500 ms, more preferably between 250 ms and 350 ms, and most preferably approximately 300 ms. By interrupting the power supply to the lamp for a short period of time, the gas present in the lamp will no longer be ionized and the built-up, possibly suboptimal, elongated, curved discharge arc will disappear. During the preceding start-up sequence, however, the wide end portions of the electrodes are slightly heated up, and both the salts and the mercury are removed from the electrodes. This leads to better conditions for the development of a short, straight discharge arc between the closely spaced end portions of the electrodes during the subsequent start-up sequence. In general, this will cause the lamp to burn better after this second start-up sequence.
Preferably, the glow frequency (fg is smaller than the ignition frequency (fi). The lower frequency causes the impedance in the circuit to be lower too, so that a higher current intensity is achieved and the electrodes heat up more rapidly.
The ignition frequency (fi) preferably ranges between 30 kHz and 120 kHz, and the ignition period (ti) preferably ranges between 0.25 and 5 seconds. More preferably, the ignition frequency (fi) is approximately 62.5 kHz, and the ignition period (ti) lasts approximately 0.5 seconds.
The glow frequency (fg) ranges between 10 kHz and 40 kHz, and the glow period (tg) preferably ranges between 0.4 s and 1.7 s. More preferably, the glow frequency (fg) is approximately 20 kHz, and the glow period (tg) lasts approximately 0.85 seconds.
Preferably, the ballast additionally comprises means capable of feeding the lamp, after the glow period (tg) of the last start-up sequence during an end glow period (tg), at an end glow frequency (fge) which is smaller than the glow frequency (fg). This end glow period serves to further increase the current intensity by further reducing the resistance of the electrodes. The end glow frequency (fge) preferably ranges between 8 kHz and 30 kHz and the end glow period (tge) preferably ranges between 0.25 and 1 second. More preferably, the end glow frequency is approximately 15.5 kHz, and the end glow period lasts approximately 0.5 s.
The operating frequency (fb) preferably ranges between 50 Hz and 200 Hz, more preferably the operating frequency is approximately 90 Hz.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.